1. Field of the Invention
The present invention relates to an active matrix organic electroluminescent device and, more particularly, to an active matrix organic electroluminescent device with a thermal insulation structure.
2. Description of the Related Art
Recently, with the development and wide application of electronic products, such as mobile phones, PDA, and notebook computers, there has been increasing demand for flat display devices which consume less electric power and occupy less space. Among flat panel displays, organic electroluminescent devices are self-emitting and highly luminous, with wider viewing angle, faster response speed, and a simple fabrication process, making them the industry display of choice.
An organic light-emitting diode (OLED) is a light-emitting diode that uses an organic electroluminescent layer as the increasingly gradually employed in flat panel displays. In accordance with driving methods, an OLED is an active matrix type (AM-OLED) or a positive matrix type (PM-OLED).
Conventionally, it is known that a positive matrix organic electroluminescent device is driven by XY matrix electrodes to display an image, employing sequential line drive. Therefore, if the number of scanning lines is in the hundreds, required instantaneous brightness is several hundred times larger than observed brightness so that electrical current passed instantaneously becomes several-hundred times larger and extreme heat is generated resulting in increasing the operating temperature of organic electroluminescent layers. However, since the aging rate of organic electroluminescent layers is in direct ratio to operating temperature thereof, the luminescent efficiency and lifetime of the organic electroluminescent device are thereby adversely affected.
The trend in organic electroluminescent display technology is for higher luminescent efficiency and longer lifetime. As a result, an active matrix organic electroluminescent device with thin film transistors is provided to solve the aforementioned problems. The active matrix organic electroluminescent device has panel luminescence with thin and lightweight characteristics, spontaneous luminescence with high luminescent efficiency and low driving voltage, and advantages of increased viewing angle, high contrast, high-response speed, flexibility and full color. As the need for larger size display devices with higher resolution grows, active matrix organic electroluminescent devices look to achieve a major market trend.
FIG. 1 is a schematic top view of a conventional active matrix organic electroluminescent device. The active matrix organic electroluminescent device comprises a plurality of pixel areas 10 arranged in a matrix form that are constituted by a plurality of data lines 12 extending along a Y direction and a plurality of source lines 14 extending along an X direction. Also, each pixel area 10 comprises two thin film transistors (TFTs) 11 and 13, a capacitor 15, an OLED 17, and two scanning lines 16 extending along the X direction.
FIG. 2 is a sectional diagram of FIG. 1 along line A–A′ showing the TFT 13 and the OLED 17. First, the TFT 13 is formed on a transparent substrate 5, wherein the TFT 13 comprises a gate electrode 31, a gate insulation layer 32, an amorphous silicon layer 33, a doped amorphous silicon layer 34, a source electrode 36, a drain electrode 35 and a silicon nitride layer 37. Next, a patterned transparent electrode 21 is formed on the transparent substrate 5, and an organic electroluminescent layer 23 and an aluminum electrode 25 are blanketly formed on the above.
The described active matrix organic electroluminescent device reduces the amount of electrical current passing the OLED 17 to prevent the organic electroluminescent layers 23 from aging from high operating temperature. However, since the active matrix organic electroluminescent device employs TFTs as driving circuits, the massive electrical current flows through the amorphous silicon layer 33 serving as a channel. A significant amount of heat is generated by the resistance of the amorphous silicon layer 33, and the operating temperature of the OLED 17 is raised by thermal conduction through the aluminum electrode 25.
FIG. 3 is a graph plotting electrical current stability against time at different operating temperatures of a common TFT. According to FIG. 3, the lifetime of TFT is unaffected by operating temperature under 50˜80° C. FIG. 4 is a graph plotting brightness against voltage at different operating temperatures of a common OLED. The luminescent efficiency of the OLED is significantly affected by increased operating temperature. High operating temperature of OLED results in accelerated aging rate of organic electroluminescent layers, and the luminescent efficiency and lifetime thereof are both lowered.
Therefore, it is necessary to develop a simple and efficient manufacturing method and structure for an active matrix organic electroluminescent device to prevent an OLED from being affected by the heat generated by a TFT.